1. Field of the Invention
This invention relates to a pigment dispersing agent used when a pigment is mixed with a coating composition in the manufacture of paints.
2. Related Art Statement
In the manufacture of paints, it is important to mix the pigment with the coating composition. In this case, the pigment to be usually used in the paint is dispersed into only a part of a vehicle constituting the paint together with a proper solvent, diluent or other additive by means of a mill used in the paint industry to obtain a dispersion paste. Then, this paste is mixed with the remaining amount of the vehicle and other necessary additives to obtain a pigment dispersing composition as a paint. However, the greatest problem of such a pigment dispersing composition lies in that the pigment particles are apt to be agglomerated. As a result, it is known to cause unfavorable phenomena such as decrease of paint stability, occurrence of troubles in the painting, reductions of color strength, gloss and distinctness of image of the finally obtained paint film, flooding, floating, color change with time and the like Such a phenomenon that the pigment particles are apt to be agglomerated is explained by London - van der Waals force attracting the particles to each other. In order to overcome this attracting force, it is necessary to provide an adsorption layer onto the surface of the particle, and consequently various methods are attempted. For instance, there are known (1) a method of dispersing the pigment by using non-ionic, anionic or cationic surfactant, or a wetting agent such as aliphatic polyvalent carboxylic acid or the like as an assistant; (2) a method of mixing the pigment with a certain substituted derivative of the pigment as disclosed in Japanese Patent laid open No. 51-18,736; (3) a method of dispersing the pigment by using a high molecular weight dispersing agent obtained by the bonding of a basic substance and a polyester compound as disclosed in Japanese Patent laid open No. 60-166,318 and No. 61-174,939; (4) a method of dispersing the pigment by using a high molecular weight dispersing agent obtained through the bonding of a basic substance and an acrylic polymer as disclosed in Japanese Patent laid open No. 46-7,294; and the like.
In the dispersion assistant used in the method (1), however, the adsorption layer adsorbed on the particle surface is thin and does not develop the satisfactory stabilizing effect and hence the pigment dispersing performances are not observed. The pigment derivative used in the method (2) is naturally colored, so that it can not generally be used for various pigments. The basic thought of the dispersing agent used in the methods (3) and (4) is a technical idea of making a block structure from a polymer portion capable of solvating with a solvent and an anchor portion adsorbed on the pigment as described by A. Topham in Progress in Organic Coatings, vol. 5, (1977) pp 237-243. In this technique, it is important that the polymer portion capable of solvating with the solvent is excellent in the compatibility with a resin added as a film forming component of the paint film and the adsorption state of the dispersing agent to the pigment is tail-like. If the polymer portion capable of solvating with the solvent is poor in the compatibility with the resin added as the film forming component of the paint film, the solvated polymer portion agglomerates to cause the degradation of pigment dispersing performances.
In the dispersing agent used in the method (3), therefore, when the compatibility between the dispersing agent and the dispersion vehicle is good, the remarkable improvement of the pigment dispersing performances is observed, but when using the dispersion vehicle having a poor compatibility, the protective layer for the dispersing agent adsorbed on the particle surface agglomerates to undesirably degrade the pigment dispersing performances. Especially, since the dispersing agent consists mainly of polyester compound, it is poor in compatibility with an acrylic resin or the like important as a painting resin, so that it is difficult to obtain good pigment dispersing performances in case of acrylic resin series paints. Furthermore, the polyester portion of this dispersing agent has no functional group reacting with a curing agent in the paint such as melamine resin, urea resin, polyisocyanate resin or the like, it acts as a plasticizer in the paint film to cause degradation of weather resistance, humidity resistance and the like in the paint film and the local floating of the pigment dispersing agent onto the surface of the paint film after the curing of the paint or so-called bleeding phenomenon, whereby the gloss and hardness are lowered. Moreover, when the dispersing agent is used for multilayer coating, is caused after trouble adhesion trouble between the paint films due to the presence of the bled component and the like.
On the other hand, the pigment dispersing agent used in the method (4) serves for ordinary acrylic resin series paints because acrylic resin is used as a polymer portion. However, the polymer portion has no reaction point such as hydroxyl group, carboxyl group or the like reacting with a crosslinking agent like the method (3) and is not chemically fixed in the paint film, so that there is a problem of damaging the quality of paint film due to the plasticizer action as previously mentioned.
Furthermore, in the method (4), all of the polymer portion is comprised of acrylic resin, so that it is difficult to be used for polyester resin series paints.
In both methods (3) and (4), since the polymer portion has no polar group such as hydroxyl group, carboxyl group or the like, it is poor in compatibility with a resin having a relatively high polarity, so that it is difficult to utilize these methods in paints consisting mainly of such a resin and hence the application range thereof is critical.